Lithium ion secondary batteries have been used for small information devices such as cellular phones or book-sized personal computers because they have a high energy density and can be driven at a high potential. All-solid-state lithium ion secondary batteries not using a flammable electrolyte are under research and development in consideration of safety. A solid electrolyte used for all-solid-state lithium ion secondary batteries is required to have a high ionic conductivity. As an oxide having a high lithium ion conductivity, Li7La3Zr2O12 having a tetragonal system garnet-related type crystal structure is known (Patent Document 1).
A solid electrolyte should have reduced grain-boundary resistance or interface resistance in order to have high ion conductivity. A solid electrolyte is therefore desirably a high density molded product. In addition, when a solid electrolyte is a high density molded product, short-circuit between positive and negative electrodes during charging/discharging process can be prevented. Further, the high density molded product can be sliced into thin pieces so that if a solid electrolyte is a high-density molded product, an all-solid-state lithium ion secondary battery can be provided as a small-sized one. Since Li7La3Zr2O12 having a garnet-related type crystal structure is however hardly sinterable, a high-density molded product cannot be prepared easily (Non-Patent Document 1).